sitemaps.org

It does e.g. not seem possible to state that two URL's return the same resource?
That is how I read the FAQ, anyway?

Several different loc elements in one url element should be the obvious way to do that.
Then again, if you were thinking like me, you would use other element names,
I'd say that one page element should contain one or more url elements.

Much less is this protocol able to say that the different sizes of the same image is indeed the same image.